Capillary electrophoresis (CE) was invented in the 1980s. A high-voltage is applied across a capillary filled with the electrolyte during the analysis so that charged molecules within the electrolyte move with different speed and in different direction depending on their electrophoretic mobility (namely, size to charge ratio of a compound), and therefore, are separated in the capillary. However, because the electroosmotic flow is always stronger than their electrophoretic mobility, all the compounds in the capillary are eventually carried to the detection end of the capillary and detected. Thus, the CE instrument can be used to separate charged molecules with different electrophoretic mobility.
The main components of CE include a separation capillary, two liquid containers at both ends of capillary, high voltage power supply, two electrodes, a detector and data output and processing device.
Hydrodynamic injection and electrophoretic injection are the two conventional injection approaches of CE. Electrophoretic injection injects the sample relying on electrophoretic mobility and (or) electroosmotic flow (EOF); but this injection mode results in bias for charged compounds and reduce the accuracy and reliability of the analysis. In addition, the two above injection methods have the following disadvantages: firstly, the high voltage across the capillary should be turned off during injection, making the established electric field interrupted and then re-established from time to time, resulting in poor precision of the analysis; secondly, both the two methods conduct the injection through “dip in” approach, which could cause cross contamination between sample and buffer solution or between different samples.
In short, the injection size of CE is very small even to nano-liter level, the traditional CE injection methods are unable to inject such small amount sample accurately. And the injection through “dip in” approach brings very poor repeatability, which makes it very difficult to achieve quota-sampling.